1. Field of the Invention
The present invention relates a pull-down detection apparatus and a pull-down detection method for detecting that an interlace video signal is generated by pull-down processing.
2. Description of Related Art
When converting commercial films recorded at 24 frames/second into an NTSC video signal at 60 fields/second, 2-3 pull-down processing that creates 5 fields from 2 frames of an original image is performed. When converting commercial films recorded at 24 frames/second into a PAL video signal at 50 fields/second and when converting commercial films recorded at 30 frames/second into an NTSC video signal at 60 fields/second, 2-2 pull-down processing that creates 2 fields from 1 frame of an original image is performed.
On the other hand, when displaying interlace video signals such as the NTSC signal at 60 fields/second and the PAL signal at 50 fields/second, interlace/progressive conversion (hereinafter referred to as the IP conversion) that converts the interlace video signal into a progressive video signal is required. The IP conversion generates a missing line of the interlace video signal to produce a progressive signal.
In the IP conversion, a method for creating pixel data of a missing line involves intra-field interpolation and inter-field interpolation. The intra-field interpolation interpolates the pixel data of a missing line from the pixel data of two lines adjacent to the missing line. The inter-field interpolation interpolates the pixel data of a missing line from the pixel data of the lines of two successive fields.
However, performing the IP conversion by the intra-field interpolation on the interlace signal generated by pull-down processing such as the 2-2 pull-down processing, which is referred to hereinafter as the pull-down signal, causes an obtained frame signal to have a lower vertical resolution than an original image before the pull-down conversion. Further, in the case of the inter-field interpolation, generating a frame signal using two fields created from different frames causes deterioration in image quality due to comb noise or the like.
In order to avoid the image quality deterioration, when performing the IP conversion on the interlace signal generated by the pull-down processing, it is preferred to create a frame signal by combining two field signals that are generated from the same frame. This allows preventing the image quality deterioration. Performing the IP conversion by using the regularity of the pull-down signal is called reverse pull-down processing.
FIG. 19 shows an example of the reverse 2-2 pull-down processing. FIG. 19 illustrates the 2-2 pull-down processing for obtaining a field signal at 60I (60 fields/second) from a frame signal at 30P (30 frames/second) and the reverse 2-2 pull-down processing for obtaining a field signal at 60P (60 frames/second) from a frame signal at 60I (60 fields/second). For example, the 2-2 pull-down processing creates a field image 1T including odd lines of the frame 1 and a field image 1B including even lines of the frame 1 from a frame 1 at 30 P. On the other hand, the reverse 2-2 pull-down processing interpolates a missing line from the field images 1T and 1B that are created from the same frame to create two frames of a frame 1-1 and a frame 1-2. This is the same as in a frame 2 and subsequent frames.
Japanese Unexamined Patent Application Publication No. 2004-242196 describes the 2-2 pull-down detection apparatus that detects that an interlace video signal is a 2-2 pull-down signal and a progressive conversion apparatus that performs IP conversion by using 2 fields created from the same frame upon detection of the 2-2 pull-down signal. FIG. 18A shows an example of the 2-2 pull-down detection apparatus disclosed therein.
The 2-2 pull-down detection apparatus 90 of FIG. 18A includes a pixel difference comparator 91, a mismatched pixel number comparator 92 and a pull-down regularity detector 93.
The pixel difference comparator 91 calculates a difference between a pixel value of a pixel b1 of a present field signal b and a pixel value of a pixel a1 of a field signal a that is an immediately following field of the present field signal b and compares the difference with a threshold value as shown in FIG. 18B. The pixels a1 and b1 are located in the positions that are perceived as substantially the same on a screen. Specifically, the pixels a1 and b1 are at the same horizontal positions and the line containing the pixel b1 is placed adjacently under the line containing the pixel a1.
If the comparison result shows that the difference in pixel value between the pixel b1 and the pixel a1 is beyond a predetermined threshold value R1, a signal set to “1” indicating a change in pixel is supplied to the mismatched pixel number comparator 92. If, on the other hand, the difference is below the threshold value R1, a signal set to “0” indicating no change in pixel is supplied to the mismatched pixel number comparator 92.
The mismatched pixel number comparator 92 receives a signal supplied from the pixel difference comparator 91, counts the number of detections of a change in pixel value by the pixel difference comparator 91 for one field period and then compares the counted number at the end of the one field with the predetermined threshold value R2. If the counted value exceeds the threshold value R2, the mismatched pixel number comparator 92 supplies a signal set to “1” indicating that the field signal a and b are generated from different frames to the pull-down regularity detector 93. If, on the other hand, the counted value falls blow the threshold value R2, it supplies a signal set to “0” indicating that the field signal a and b are generated from the same frame to the pull-down regularity detector 93.
If the output signal from the mismatched pixel number comparator 92 has a pattern that 1 and 0 are repeated alternately like “1010 . . . ” or “0101 . . . ”, the pull-down regularity detector 93 determines that there is a regularity of 2-2 pull-down signal. If, on the other hand, that repeated pattern of the output signal from the mismatched pixel number comparator 92 is lost, the pull-down regularity detector 93 determines that no regularity of 2-2 pull-down signal exists.
As described above, the pull-down detection apparatus determines if an image changes between adjacent fields and observes the regularity of the determination results, thereby detecting if it is a pull-down signal. Thus, for the accurate detection of the pull-down signal, it is necessary for the 2-2 pull-down detection apparatus 90 shown in FIG. 18A, for example, to perform accurate determination of a change in unit of pixels by the pixel difference comparator 91 and accurate determination of a change in units of fields by the mismatched pixel number comparator 92.
The conventional pull-down detection apparatus such as the 2-2 pull-down detection apparatus 90 shown in FIG. 18A detects a change in image based on the result of pixel comparison in time and vertical directions with use of the pixel of the present field b and the pixel of the subsequent field a. However, the conventional comparison is subject to wrong determination of an image without change as an image with change particularly when a display image contains a high-frequency portion (hereinafter referred to as the edge portion) such as an oblique line and an object boundary, for example.
For example, when a change in pixel value is large in the horizontal direction, it means that a display image contains a high-frequency portion (edge portion) such as an oblique line and an object boundary. The edge portion is likely to have an edge portion with a high vertical frequency in the vertical direction as well. In this case, the conventional pull-down detection apparatus is likely to wrongly determines that a change occurs in the image in spite that there is no change in image between the pixel field and the subsequent field due to a difference in pixel value caused by the edge portion.
Further, in a pixel where a change in pixel value is excessively large between the subsequent field a and the present field b, which is, where a temporal change is large, even if the conventional pull-down detection apparatus detects a change in pixel value, it is unable to determine if it is due to a change in image between fields or due to the edge portion of a high-frequency image. If such a pixel is added to the determination of an image change, the wrong determination of image change is likely to occur even when the subsequent field a and the present field b are created from the same frame.
As described in the foregoing, the present invention has recognized that the conventional pull-down detection apparatus is likely to wrongly determine an image without image change to be an image with image change, thus having lower detecting accuracy of a pull-down signal.